Conventionally, the vapor deposition by means of gas-phase plasma as a film-forming method using plasma has been heretofore carried out widely. However, in the gas-phase plasma, since a raw material is supplied in a state of gas, the substance density has been low, and it has been difficult to improve the film-forming rate. Hence, a film-forming method using in-liquid plasma has been attracting attention, film-forming method in which a raw material can be supplied in a state of liquid with high substance density by generating plasma in the liquid.
Regarding the in-liquid plasma, in Patent Literature No. 1 (Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2003-297, 598) and Patent Literature No. 2 (Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2004-152,523), bubbles are generated by using ultrasonic wave onto dodecane (C12H26) that exists in a liquid state at ordinary temperature and under ordinary pressure, and additionally electromagnetic wave is irradiated onto the position at which the bubbles are generating, thereby generating plasma with high energy within the bubbles. At this time, the dodecane exists in a state of gas inside the bubbles, and is excited by means of the plasma. And, by means of contacting the bubbles including the plasma with a substrate, carbon is deposited onto a surface of the substrate, and then an amorphous carbon film is formed at high rate.
In the aforementioned method in which in-liquid plasma is generated by irradiating electromagnetic wave onto bubbles that have generated in liquid, high reaction rate is obtainable because molecular density is extremely high in liquid phase compared with that in gas phase. However, in liquid that exhibits electric conductivity, such as water and alcohol, there is such a problem that eddy currents occur in the liquid and thereby the energy of the irradiated electromagnetic wave has been consumed. Moreover, there is such a problem as well that the electromagnetic wave has attenuated because the hydroxyl group and the like absorb specific frequencies.
Hence, in Example No. 2 of Patent Literature No. 3 (International Publication Pamphlet No. 2006/059808), plasma is generated in ethanol using an electrode for in-liquid plasma, electrode which has electrically-conducting member having a discharging end surface that contacts with the ethanol, and an insulating member covering the outer periphery of the electrically-conducting member excepting the discharging end surface. When supplying high-frequency electricity to the electrically-conducting member, the ethanol boils and then bubbles generate because the leading end of the electrically-conducting member generates heat. At the same time, plasma generates inside the bubbles because high-frequency wave is irradiated onto the position at which the bubbles generate. These bubbles are contacted with an electrically-conducting substrate (or a second electrode) that is disposed to face the discharging end surface, and thereby an amorphous carbon film is formed on a surface of the substrate.
When using the electrode for in-liquid plasma that is disclosed in Patent Literature No. 3, plasma generates readily in a broader range of liquids including liquids with electric conductivity, such as water and alcohol.
However, in order to generate plasma by supplying high-frequency electricity to an electrically-conducting member, making use of the second electrode that faces the discharging end surface of the electrically-conducting member is essential. That is, even when an electrically-conducting member is disposed alone in liquid and then high-frequency electricity is supplied to the electrically-conducting member, no plasma generates.
Moreover, in Example No. 2 of Patent Literature No. 3, by making use of a substrate having electric conductivity as the second electrode, an amorphous carbon film is formed on a surface of the substrate. Therefore, the film forming onto a surface of the substrate with electric conductivity is done with ease. However, it is not easy to carry out film forming to substrates having no electric conductivity, such as ceramic.
In addition, depending on film-forming conditions, not only glow discharge that is suitable for film forming, but also arc discharge are likely to occur between the electrically-conducting member and the substrate (or the second electrode). When arc discharge generates, it is not desirable because the substrate has deteriorated, or discharge marks have remained on the resulting film's surface. Moreover, there might arise cases where no coated film having desired property is formed due to the influence of arc discharge.